Variable in synchronized hashtable seems not thread-safe #13

I have a variable: $syncHash.count, which is supposed to be thread-safe. But in this case, it's not:
$collection | Invoke-Parallel -scriptblock {
$syncHash.count ++
}
I have to use mutex to protect it, in order to get correct count:
$collection | Invoke-Parallel -scriptblock {
$syncHash.mutex.WaitOne()
$syncHash.count ++
$syncHash.mutex.ReleaseMutex()
}

Is this normal?